1. Field of the Invention
The invention relates to a projector for exposing a photosensitive substrate, suitable for use when a circuit pattern is to be transferred in fabricating a semiconductor integrated circuit.
2. Description of the Related Art
A projector for exposing a photosensitive substrate is required to have high productivity in processing micro patterns for a semiconductor integrated circuit. When a circuit pattern is to be fabricated on a semiconductor, a photolithography technology is generally used for transferring a pattern formed on a mask onto a substrate. In the photolithography, a mask on which a desired pattern is formed is irradiated with exposure light such as ultra violet ray, and thereby images of the pattern are projected through a projection optical system onto a photosensitive resist layer formed on a substrate. The resolution and depth of focus of a projector is dependent on a number of apertures (hereinafter referred to as "NA") and a wavelength of exposure light. A larger number of apertures (NA) and/or a shorter wavelength of exposure light enhance resolution of a projector, but decrease depth of focus.
One of exposure methods for increasing or deepening depth of focus while preventing a resolution of a projector from deteriorating has been suggested in Japanese Patent Public Disclosure No. 4-136854 published on May 11, 1992. This Disclosure sets forth a phase shifting method in which a mask has an exposure pattern composed of half-transparent material which causes phase differences among lights passing therethrough. The phase shifting method utilizes diffraction of light for enhancing a resolution of an optical system for projection, and includes a phase shifter, namely a dielectric substance for inverting a phase of exposure light every other light-pervious pattern through which the exposure light passes.
Another method for increasing depth of focus has been suggested in Japanese Patent Public Disclosure No. 4-180612 published on Jun. 26, 1992. This Disclosure sets forth a device for dividing a light flux, disposed in an optical path of an optical system for illumination. Hereinafter this method is referred to as a light flux dividing method.
The above-mentioned phase shifting method can enhance depth of focus of an isolated pattern in which adjacent apertures are spaced far from each other, but decreases the contrast of optical images and thereby disables resolution of a projector in a pattern in which adjacent apertures are relatively close to each other. Thus, the phase shifting method is useful only for an isolated pattern and hence has been used only for contact hole exposure. Conventionally, in a mask including contact holes, since adjacent holes are spaced apart from each other by a distance more than double hole diameter, such contact holes can be deemed to constitute an isolated pattern. However, the necessity of a mask as disclosed in FIG. 6 has recently increased for enhancing integration of a semiconductor integrated circuit. The mask illustrated in FIG. 6 comprises a glass substrate 4 and an exposure pattern 5 formed on the substrate 4. The pattern 5 includes both an isolated pattern 9 and an arranged pattern 10 in which a distance between adjacent holes is smaller than double hole diameter. If the depth of focus of the isolated pattern 9 is intended to increase by means of the abovementioned phase shifting method, there arises a problem that the arranged pattern 10 cannot be resolved.
FIG. 7 illustrates another mask pattern comprising an isolated line pattern 11 and an arranged line pattern 12. There would arise the same problem as that aforementioned with reference to FIG. 6, if the depth of focus of the isolated pattern 11 is intended to increase by means of the phase shifting method.
The light flux dividing method disclosed in Japanese Public Disclosure No. 4-180612 could increase the depth of focus of the arranged patterns 10 and 12, but could not increase the depth of focus of the isolated patterns 9 and 11. Thus, the light flux dividing method could not increase the depth of focus of all the patterns 9, 10, 11 and 12 formed on the masks as illustrated in FIGS. 6 and 7.